[CMake] Make sure _cmake_system_name has a default

[daltenty]

Summary

We currently try to pick it up from the CMake arguments passed to llvm_ExternalProject_Add but
if there isn't an explicit option passed, we should reflect CMake's own default behaviour
of targeting the host, since we'll make decisions about what tools to use for the build based on
the setting. Otherwise, we'll get different behaviour between configuring an external project with
the default target and configuring with an explicit one targeting the same platform.

[hubert.reinterpretcast]

llvm/cmake/modules/LLVMExternalProjectUtils.cmake

55

Capitalize as a sentence. Also, is this a statement about the behaviour of CMake (and we're just reflecting that locally through _cmake_system_name) or a statement about what this code is causing CMake to do? Please reword to clarify.

I would also suggest some editing of the patch description.

[daltenty]

• Update comment

[hubert.reinterpretcast]

LGTM; thanks.

[stevewan]

This is consistent with the CMAKE default behaviour. LGTM. Let's see if other reviewers have further input about the updated comment.

[phosek]

This broke our macOS builders because now llvm-libtool-darwin tool is being used by default which leads to a build failure:

Merging archives: ['/b/s/w/ir/x/w/staging/llvm_build/lib/libc++abi.a', '/b/s/w/ir/x/w/staging/llvm_build/lib/libunwind.a']
Command: '/b/s/w/ir/x/w/staging/llvm_build/bin/llvm-libtool-darwin' '-static' '-o' '/b/s/w/ir/x/w/staging/llvm_build/lib/libc++abi.a' '-s' 'cxa_aux_runtime.cpp.o' 'cxa_default_handlers.cpp.o' 'cxa_demangle.cpp.o' 'cxa_exception_storage.cpp.o' 'cxa_guard.cpp.o' 'cxa_handlers.cpp.o' 'cxa_vector.cpp.o' 'cxa_virtual.cpp.o' 'stdlib_exception.cpp.o' 'stdlib_stdexcept.cpp.o' 'stdlib_typeinfo.cpp.o' 'abort_message.cpp.o' 'fallback_malloc.cpp.o' 'private_typeinfo.cpp.o' 'stdlib_new_delete.cpp.o' 'cxa_exception.cpp.o' 'cxa_personality.cpp.o' 'libunwind.cpp.o' 'Unwind-EHABI.cpp.o' 'Unwind-seh.cpp.o' 'Unwind_AppleExtras.cpp.o' 'UnwindLevel1.c.o' 'UnwindLevel1-gcc-ext.c.o' 'Unwind-sjlj.c.o' 'UnwindRegistersRestore.S.o' 'UnwindRegistersSave.S.o'
Exit Code: 1
Standard Error:
--
llvm-libtool-darwin: Unknown command line argument '-s'.  Try: '/b/s/w/ir/x/w/staging/llvm_build/bin/llvm-libtool-darwin --help'
llvm-libtool-darwin: Did you mean '-D'?
--

[phosek]

In D88157#2299255, @phosek wrote:

This broke our macOS builders because now llvm-libtool-darwin tool is being used by default which leads to a build failure:

Merging archives: ['/b/s/w/ir/x/w/staging/llvm_build/lib/libc++abi.a', '/b/s/w/ir/x/w/staging/llvm_build/lib/libunwind.a']
Command: '/b/s/w/ir/x/w/staging/llvm_build/bin/llvm-libtool-darwin' '-static' '-o' '/b/s/w/ir/x/w/staging/llvm_build/lib/libc++abi.a' '-s' 'cxa_aux_runtime.cpp.o' 'cxa_default_handlers.cpp.o' 'cxa_demangle.cpp.o' 'cxa_exception_storage.cpp.o' 'cxa_guard.cpp.o' 'cxa_handlers.cpp.o' 'cxa_vector.cpp.o' 'cxa_virtual.cpp.o' 'stdlib_exception.cpp.o' 'stdlib_stdexcept.cpp.o' 'stdlib_typeinfo.cpp.o' 'abort_message.cpp.o' 'fallback_malloc.cpp.o' 'private_typeinfo.cpp.o' 'stdlib_new_delete.cpp.o' 'cxa_exception.cpp.o' 'cxa_personality.cpp.o' 'libunwind.cpp.o' 'Unwind-EHABI.cpp.o' 'Unwind-seh.cpp.o' 'Unwind_AppleExtras.cpp.o' 'UnwindLevel1.c.o' 'UnwindLevel1-gcc-ext.c.o' 'Unwind-sjlj.c.o' 'UnwindRegistersRestore.S.o' 'UnwindRegistersSave.S.o'
Exit Code: 1
Standard Error:
--
llvm-libtool-darwin: Unknown command line argument '-s'.  Try: '/b/s/w/ir/x/w/staging/llvm_build/bin/llvm-libtool-darwin --help'
llvm-libtool-darwin: Did you mean '-D'?
--

D88449 should address this issue.

[daltenty]

In D88157#2299284, @phosek wrote:

In D88157#2299255, @phosek wrote:

This broke our macOS builders because now llvm-libtool-darwin tool is being used by default which leads to a build failure:

D88449 should address this issue.

Sorry for the breakage.

llvm/cmake/modules/LLVMExternalProjectUtils.cmake

function(llvm_ExternalProject_Add name source_dir)
   canonicalize_tool_name(${name} nameCanon)
 
   foreach(arg ${ARG_CMAKE_ARGS})
     if(arg MATCHES "^-DCMAKE_SYSTEM_NAME=")
       string(REGEX REPLACE "^-DCMAKE_SYSTEM_NAME=(.*)$" "\\1" _cmake_system_name "${arg}")
     endif()
   endforeach()
 
+  # If CMAKE_SYSTEM_NAME is not set explicitly in the arguments passed to us,
+  # reflect CMake's own default.
+  if (NOT _cmake_system_name)
+    set(_cmake_system_name "${CMAKE_HOST_SYSTEM_NAME}")
+  endif()
+
   if(NOT ARG_TOOLCHAIN_TOOLS)
     set(ARG_TOOLCHAIN_TOOLS clang lld llvm-ar llvm-ranlib llvm-nm llvm-objdump)
     if(_cmake_system_name STREQUAL Darwin)
       list(APPEND ARG_TOOLCHAIN_TOOLS llvm-libtool-darwin llvm-lipo)
     elseif(_cmake_system_name STREQUAL Windows)
       list(APPEND ARG_TOOLCHAIN_TOOLS llvm-lib)
     else()
       # TODO: These tools don't fully support Mach-O format yet.